Multi-coordinate scanning systems are used as in multi-coordinate measuring machines for measuring test specimens so in machine tools for measuring workpieces. In machine tools, the scanning systems may be used for measuring workpieces because of the similarity of the mechanical structure and the kinematics of the numerically controlled machine tools and the multi-coordinate measuring machines. For use in the machine tools, a suitable three-dimensional scanning system should be appropriately adapted in both mechanical and signalization respects. A scanning system is located, instead of a tool, in a tool-exchange magazine and is replaced and positioned in the same manner as the tool. After the signalization system of the scanning system has been linked with the path measuring system of the machine tool, then the machine tool can be generally used as a coordinate measuring machine. The coordinate system of the machine tool, which serves as a spacial reference base, together with the path measuring system and the electronics of the three-coordinate scanning system, enable spacial measurements. However, a machine tool only then will be able to effect spatial measurements of a workpiece when the system geometrical errors of the spacial reference base, in particular, the displacement errors of the machine tool and the system errors of the measuring standard are determined and compensated during the measuring process. To this end, individual errors of the machine tool are determined with a suitable comparison measuring system, e.g., laser-interferometer, and are stored in the control system in the form of a correction matrix. In this case, the triplet of the coordinate of the path measuring system is processed together with the stored correction values. A microprocessor, which is integrated into machine tool control or is adapted for effecting same, converts the scanning coordinates into actual measurement of the work-piece, effects the calculation of the set-actual value deviation and transmits the results to peripheral units and to the last stage of calculation for providing corrected NC-data for compensating the machining errors.
For determination of the corrected NC-data, the parameters of the three-dimensional scanning system should be take into account.
An essential element of the scanning system is a spring-biased tracer pin which can be displaced along a predetermined path. If the displacement of the feeler ball can be registered within the spacer pin stroke, then a measuring scanning system becomes available.
After each spacer pin exchange, calibration should take place because, due to the mechanics of the scanning system, the measuring forces in the directions of different coordinates are different and, therefore, the tracer pin length and its deformation during the measuring process should be compensated. In particular, the calibration becomes necessary because of different spring rates, which determine the force/path relationship during the displacement of the tracer pin.
Accordingly, the object of the invention is a method of and an apparatus for effecting calibration, which facilitate the calibration process and improves the results of the calibration process.